1. Click the Apple in upper left corner
  2. Click System Preferences
  3. Click Network,
  4. Be sure Airport is on and click its Advanced button. The window should show Preferred and the available wifi connections should be shown with signal strengths.
  5. If it says Automatic, click the box down arrow and click Preferred, then select the best or assigned connection.
  6. If there is a box for Remember any network this computer has joined, be sure it is checked.

Paul Ide